1. Field of the Invention
The present invention relates to a mold for molding a foamed tube for a tire which is used in place of a hollow rubber tube of a pneumatic tire, and to a method of manufacturing a foamed tube for a tire.
2. Description of the Related Art
For example, in order to prevent bursting of tires mounted to motorcycles for motocross, there are cases in which a donut-shaped toroidal sponge, which is formed from an elastic body of rubber or the like, is filled in the tire in place of an air tube.
Conventionally, such a toroidal sponge is manufactured as follows.    (1) First, a rubber composition containing foaming agents, foaming auxiliaries and the like, is extruded at an extruder so as to form a solid cylindrical rubber material. Both ends thereof are joined together and a donut-shaped torus 14 such as shown in FIG. 2 is molded.    (2) Next, by using a mold 102 formed from an upper die 102A and a lower die 102B such as shown in FIG. 12A, the donut-shaped torus 14 is filled into the mold 102 as shown in FIG. 12A. After the upper die 102A and the lower die 102B are fixed by an unillustrated fastening hardware, the mold 102 is sandwiched between heating plates and vulcanization is carried out.
In the conventional mold 102, a joining face 103A of the upper die 102A and a joining face 103B of the lower die 102B are both horizontal.
After vulcanization, when the mold 102 is removed from the heating plates and the fastening hardware is undone, the donut-shaped torus 14 expands, and a sponge-like foamed tube for a tire is obtained.
The foamed tube for a tire which is obtained in this way is cooled naturally at room temperature, and is kept until the foaming rate decreases and is stable. A foamed tube for a tire whose dimensions have stabilized is then served for use (i.e., actually mounted to a motorcycle or the like).
However, after vulcanization, when the mold is opened (i.e., when the mold is opened to the atmosphere), the gas generated in the rubber becomes air bubbles and expands. The rubber composition suddenly expands (the volume thereof suddenly increases to about 500 to 1200%) substantially simultaneously with the opening of the mold 102. The swelled rubber composition enters into the narrow gap between the joining faces of the upper die 102A and the lower die 102B which are opened slightly, and bursts out to the exterior of the mold 102.
At this point in time, the expanding of the rubber composition is local, and the elongation of the outer surface at this swelled portion is extremely high. Thus, as shown in FIG. 13, tears 104, 106 arise in the surface of the rubber composition which has burst out and swelled. As shown in FIG. 14, a problem arises in that there are many cases in which the tears 104, 106 remain in the foamed tube for a tire (the donut-shaped torus 14) which is finally obtained. Thus, conventionally, the speed of opening the vulcanizing press to which the mold 102 is mounted is made to be extremely fast in order to handle the expanding of the rubber composition, or the joining face 103A and the joining face 103B of the upper die 102A and the lower die 102B are made to be extremely wide so as to suppress the bursting out of the rubber composition to the exterior of the mold, or the foaming rate is set to be low to the extent that tears do not arise, or the blending composition of the rubber composition is adjusted so as to make the modulus high.
However, making the opening speed of the mold 102 be as fast as the speed at which the rubber composition expands results in the vulcanizing press becoming a large-sized equipment. The cost thereof then increases drastically, and a large space is required for the placement thereof.
On the other hand, making the horizontal mating surfaces of the mold 102 wide leads to the outer dimensions of the vulcanization mold becoming greater. The cost of the mold 102 then inevitably increases, the handling thereof is difficult and workability is poor.
Further, in order to address the above problem by adjusting the blending composition of the rubber composition, the properties of the rubber composition have to be restricted, and there are cases in which desired product properties cannot be obtained.